His dark matter behaves
in accordance with MOND precisely in the regime where MOND is known
to work and behaves like conventional cold dark matter in exactly
those regimes where those models work.

"You need to have
some phase transition that makes dark matter behave differently
at a shorter scale than a longer scale," said Stefano Liberati,
a physicist at the International School for Advanced Studies in
Trieste, Italy, who has also worked on superfluid dark matter
models. Khoury's model does that.

The term was first
invoked nearly 80 years ago by the astronomer Fritz Zwicky,
who realized that some unseen gravitational force was needed to stop
individual galaxies from escaping giant galaxy clusters.

Later, Vera Rubin
and Kent Ford used unseen dark matter to explain why galaxies
themselves don't fly apart. Yet even though we use the term "dark
matter" to describe these two situations, it's not clear that the
same kind of stuff is at work.

The simplest and most
popular model holds that dark matter is made of weakly interacting
particles that move about slowly under the force of gravity. This
so-called "cold" dark matter accurately describes large-scale
structures like galaxy clusters.

However, it doesn't do a
great job at predicting the rotation curves of individual galaxies.
Dark matter seems to act differently at this scale.

In the latest
effort to resolve this conundrum, two physicists have
proposed
that dark matter is capable of changing phases at different size
scales.

Justin Khoury, a physicist at the University of Pennsylvania,
and his former postdoc Lasha Berezhiani, who is now at Princeton University, say that
in the cold, dense environment of the galactic halo, dark matter
condenses into a superfluid - an exotic quantum state of matter that
has zero viscosity.

Yet at the scale of
galaxy clusters, the special conditions required for a superfluid
state to form don't exist; here, dark matter behaves like
conventional cold dark matter.

"It's a neat
idea," said
Tim Tait,
a particle physicist at the University of California, Irvine.
"You get to have two different kinds of dark matter described by
one thing."

And that neat idea
may soon be testable.

Although other
physicists have toyed with similar ideas, Khoury and Berezhiani are
nearing the point where they can extract testable predictions that
would allow astronomers to explore whether our galaxy is swimming in
a superfluid sea.

Impossible Superfluids

Here on Earth,
superfluids aren't exactly commonplace.

But physicists have
been cooking them up in their labs since 1938. Cool down particles
to sufficiently low temperatures and their quantum nature will start
to emerge.

Their matter waves
will spread out and overlap with one other, eventually coordinating
themselves to behave as if they were one big "superatom."

They will become
coherent, much like the light particles in a laser all have the same
energy and vibrate as one. These days even undergraduates create
so-called Bose-Einstein condensates (BECs) in the lab, many of which
can be classified as superfluids.

Superfluids don't
exist in the everyday world - it's too warm for the necessary
quantum effects to hold sway.

Because of that,

"probably ten
years ago, people would have balked at this idea and just said
'this is impossible'," said Tait.

But recently, more
physicists have warmed to the possibility of superfluid phases
forming naturally in the extreme conditions of space.

Superfluids may
exist inside neutron stars, and some researchers have speculated
that space-time itself may be a superfluid.

So why shouldn't
dark matter have a superfluid phase, too?

To make a
superfluid out of a collection of particles, you need to do two
things:

pack the particles together at very high densities

cool
them down to extremely low temperatures

In the lab,
physicists (or undergraduates) confine the particles in an
electromagnetic trap, then zap them with lasers to remove the
kinetic energy and lower the temperature to just above absolute
zero.

Lucy Reading-Ikkanda

Quanta Magazine

Inside galaxies,
the role of the electromagnetic trap would be played by the galaxy's
gravitational pull, which could squeeze dark matter together enough
to satisfy the density requirement.

The temperature
requirement is easier:

Space, after all, is naturally cold.

Justin Khoury,

a physicist at the
University of Pennsylvania,

co-developed the dark
matter superfluid model.

Perimeter Institute

Outside of the "halos" found in the immediate vicinity of galaxies,
the pull of gravity is weaker, and dark matter wouldn't be packed
together tightly enough to go into its superfluid state.

It would act as
dark matter ordinarily does, explaining what astronomers see at
larger scales.

But what's so special about having dark matter be a superfluid?

How
can this special state change the way that dark matter appears to
behave?

A number of
researchers over the years have played with similar ideas.

But Khoury's approach is unique because it shows how the superfluid
could give rise to an extra force.

In physics, if you
disturb a field, you'll often create a wave. Shake some electrons -
for instance, in an antenna - and you'll disturb an electric field
and get radio waves.

Wiggle the
gravitational field with two colliding black holes and you'll
create gravitational waves. Likewise, if you poke a superfluid,
you'll produce phonons - sound waves in the superfluid itself.

These phonons give
rise to an extra force in addition to gravity, one that's analogous
to the electrostatic force between charged particles.

"It's nice
because you have an additional force on top of gravity, but it
really is intrinsically linked to dark matter," said Khoury.
"It's a property of the dark matter medium that gives rise to
this force."

The extra force
would be enough to explain the puzzling behavior of dark matter
inside galactic halos.

A Different Dark Matter
Particle

Their efforts have
focused on so-called weakly interacting massive particles, or WIMPs.

WIMPs have been popular because not only would the particles account
for the majority of astrophysical observations, they pop out
naturally from hypothesized extensions of the Standard Model of
particle physics.

Yet no one has ever
seen a WIMP, and those hypothesized extensions of the Standard Model
haven't shown up in experiments either, much to physicists'
disappointment.

With each new null
result, the prospects dim even more, and physicists are increasingly
considering other dark matter candidates.

"At what point
do we decide that we've been barking up the wrong tree?" said
Stacy
McGaugh, an astronomer at Case Western Reserve University.

The dark matter
particles that would make Khoury and Berezhiani's idea work are
emphatically not WIMP-like.

WIMPs should be
pretty massive as fundamental particles go - about as massive as 100
protons, give or take. For Khoury's scenario to work, the dark
matter particle would have to be a billion times less massive.

Consequently, there
should be billions of times as many of them zipping through the
universe - enough to account for the observed effects of dark matter
and to achieve the dense packing required for a superfluid to form.
In addition, ordinary WIMPs don't interact with one another.

It's just a matter
of determining whether that interaction is weak or strong.

Cosmic Superfluid
Searches

The next step for
Khoury and Berezhiani is to figure out how to test their model - to
find a telltale signature that could distinguish this superfluid
concept from ordinary cold dark matter.

One possibility:

dark matter vortices.

In the lab, rotating superfluids give rise to
swirling vortices that keep going without ever losing energy.
Superfluid dark matter halos in a galaxy should rotate sufficiently
fast to also produce arrays of vortices.

If the vortices
were massive enough, it would be possible to detect them directly.

Small vortices form in a swirling bath
of superfluid helium.
Nonlinear Dynamics
Lab at the University of Maryland

Unfortunately, this
is unlikely to be the case:

Khoury's most
recent computer simulations suggest that vortices in the dark
matter superfluid would be "pretty flimsy," he said, and
unlikely to offer researchers clear-cut evidence that they
exist.

He speculates it
might be possible to exploit the phenomenon of gravitational lensing
to see if there are any scattering effects, similar to how a crystal
will scatter X-ray light that passes through it.

Astronomers could
also search for indirect evidence that dark matter behaves like a
superfluid.

Here, they'd look to galactic mergers. The rate that
galaxies collide with one another is influenced by something called
dynamical friction.

Imagine a massive
body passing through a sea of particles. Many of the small particles
will get pulled along by the massive body. And since the total
momentum of the system can't change, the massive body must slow down
a bit to compensate.

That's what happens
when two galaxies start to merge.

If they get sufficiently close,
their dark matter halos will start to pass through each other, and
the rearrangement of the independently moving particles will give
rise to dynamical friction, pulling the halos even closer.

The effect helps
galaxies to merge, and works to increase the rate of galactic
mergers across the universe.

But if the dark
matter halo is in a superfluid phase, the particles move in sync.
There would be no friction pulling the galaxies together, so it
would be more difficult for them to merge.

This should leave
behind a telltale pattern:

rippling interference patterns in how
matter is distributed in the galaxies.

Perfectly Reasonable
Miracles

While Stacy McGaugh is
mostly positive about the notion of superfluid dark matter, he
confesses to a niggling worry that in trying so hard to combine the
best of both worlds, physicists might be creating what he terms a "Tycho
Brahe solution."

The 16th-century
Danish astronomer invented a hybrid cosmology in which the Earth was
at the center of the universe but all the other planets orbited the
sun.

It attempted to
split the difference between the ancient Ptolemaic system and the
Copernican cosmology that would eventually replace it.

"I worry a
little that these kinds of efforts are in that vein, that maybe
we're missing something more fundamental," said McGaugh. "But I
still think we have to explore these ideas."

Tim Tait admires this
new superfluid model intellectually, but he would like to see the
theory fleshed out more at the microscopic level, to a point where,

"we can really
calculate everything and show why it all works out the way it's
supposed to. At some level, what we're doing now is invoking a
few miracles" in order to get everything to fit into place, he
said.

One potential
sticking point is that Khoury and Berezhiani's concept requires a
very specific kind of particle that acts like a superfluid in just
the right regime, because the kind of extra force produced in their
model depends upon the specific properties of the superfluid.

They are on the
hunt for an existing superfluid - one created in the lab - with
those desired properties.

"If you could
find such a system in nature, it would be amazing," said Khoury,
since this would essentially provide a useful analog for further
exploration.

"You could in
principle simulate the properties of galaxies using cold atoms
in the lab to mimic how superfluid dark matter behaves."